How it works
The position sizing formula
Lot size is the money you accept to lose divided by what you lose per lot if price travels to your stop: lots = (balance × risk %) ÷ (stop pips × pip value per lot).
Pip value per lot is the contract size × pip size, converted into your account currency. On EUR/USD with a USD account that is 10 USD. If the quote currency is not your account currency (for example USD/JPY on a USD account) it must be converted using the current exchange rate.
Why it rounds down
Brokers accept lots in fixed steps (usually 0.01). Rounding up would make you risk more than planned, so the calculator always rounds down and shows the resulting real risk.
FormulaLots = (Balance × Risk %) ÷ (Stop in pips × Pip value per lot)